VisibleDeprecationWarning when using `set_format("numpy")`
Code to reproduce:

```
from datasets import load_dataset
dataset = load_dataset("glue", "mnli")

from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained('distilbert-base-cased')

def tokenize_function(dataset):
    return tokenizer(dataset['premise'])

tokenized_datasets = dataset.map(tokenize_function, batched=True, remove_columns=dataset['train'].features)

tokenized_datasets.set_format("numpy")

tokenized_datasets['train'][5:8]
```
Outputs:

```
python3.9/site-packages/datasets/formatting/formatting.py:167: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray
  return np.array(array, copy=False, **self.np_array_kwargs)
```